The new release isn’t just a version port; it’s also got substantial new functionality. This includes a good deal of performance work, better javascript debugging, and a redesigned console. I’ve been using the 1.2 alpha series, and this beta feels very stable to me; if you’re doing any work in Firefox 3 it’s definitely worth grabbing.

Keith – what’s disabled by default is the “Net” and “Script” tabs, which the developers say are the ones that hurt performance. I suspect (without having benchmarked it) that hooking into Firefox’s default notification of script errors is essentially free.

My only complaint with the new version of Firebug is there’s no way to disbale it. The devs say that it’s disabled by default, but at least for me, it was always on (as evidenced by a constant red “x Errors” in the status bar) , with no way to disable it (and free up the cpu cycles it was using) except to disable the plugin via the Addons menu.